A known type of brake device for a motor vehicle has a friction element or “brake pad” suitable or being pressed under the action of a piston against a brake disk that is constrained to rotate with a wheel of the vehicle.
The piston is in contact with a rear face of the brake pad in a plane that is substantially parallel to the plane of the brake disk.
The piston is moved towards the brake disk, e.g. by means of a hydraulic fluid under pressure or by means of an electric motor.
Document WO 98/14715 describes an electrical disk brake including an electric motor for moving a piston towards a brake pad and for pressing the brake pad against the brake disk when the brakes are applied. The bearing plane in which the piston bears against the brake pad forms a non-zero angle with the plane containing the brake disk along a radius of the brake disk, so that the bearing plane is tilted in the direction in which the brake disk rotates when the motor vehicle is traveling in the forward direction.
Thus, when the brakes are applied, while the brake pad is being pressed against the brake disk, the braking force delivered by the electric motor is amplified by a wedging effect to which the pad is subjected between the brake disk and the piston, thereby offering assistance to braking. The wedging effect is due to the torque exerted on the pad by the disk due to said disk rotating, with the pad then being clamped between the disk and the face of the piston that bears against the slanting face of the pad.
The wedging effect, which is well known the in the field of brakes, makes it possible to amplify very considerably the braking force applied to the piston. Unfortunately there is a risk of that phenomenon causing the wheel to block, which is detrimental to the stability of the vehicle. In addition, such amplification is not necessary for low levels of braking, and yet, in the example described in Document WO 98/14715, the amplification is present each time the brakes are applied.